The ongoing third generation partnership project (3GPP) Rel-13 study item, “Licensed-Assisted Access” (LAA), intends to allow long term evolution (LTE) equipment to also operate in the unlicensed 5 giga-Hertz (GHz) radio spectrum. The unlicensed 5 GHz spectrum is used as a complement to the licensed spectrum. Accordingly, devices connect in the licensed spectrum (primary cell or PCell) and use carrier aggregation to benefit from additional transmission capacity in the unlicensed spectrum (secondary cell or SCell). To reduce the changes required for aggregating licensed and unlicensed spectrum, the LTE frame timing in the primary cell is simultaneously used in the secondary cell.
Regulatory requirements, however, may not permit transmissions in the unlicensed spectrum without prior channel sensing. Since the unlicensed spectrum must be shared with other radios of similar or dissimilar wireless technologies, a so called listen-before-talk (LBT) method needs to be applied. LBT involves sensing the medium for a pre-defined minimum amount of time and backing off if the channel is busy. Today the unlicensed 5 GHz spectrum is mainly used by equipment implementing the Institute of Electrical and Electronic Engineers (IEEE) 802.11 Wireless Local Area Network (WLAN) standard. This standard is also known under its marketing brand, “Wi-Fi.”
The choice of parameters used in the LBT procedure prior to accessing the channel has a major impact on inter-radio access technology (RAT) coexistence and throughput. Of particular relevance is the method of adapting the size of contention windows in different random access methods, which determines how long nodes have to wait before attempting to transmit on the medium.
LTE uses orthogonal frequency division multiplex (OFDM) in the downlink and discrete Fourier transform (DFT)-spread OFDM (also referred to as single-carrier frequency division multiple access (FDMA)) in the uplink. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 1, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval. The uplink subframe has the same subcarrier spacing as the downlink and the same number of sub carrier (SC)-FDMA symbols in the time domain as OFDM symbols in the downlink.
In the time domain, LTE downlink transmissions are organized into radio frames of 10 ms, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 ms as shown in FIG. 2. Each subframe comprises two slots of duration 0.5 ms each, and the slot numbering within a frame ranges from 0 to 19. For normal cyclic prefix, one subframe consists of 14 OFDM symbols. The duration of each symbol is approximately 71.4 μs.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks (RB), where a resource block corresponds to one slot (0.5 ms) in the time domain and 12 contiguous subcarriers in the frequency domain. A pair of two adjacent resource blocks in time direction (1.0 ms) is known as a resource block pair. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth.
Downlink transmissions are dynamically scheduled, i.e., in each subframe the base station transmits control information about which wireless device data is transmitted to and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe and the number n=1, 2, 3 or 4 is known as the Control Format Indicator (CFI). The downlink subframe also contains common reference symbols, which are known to the receiver and used for coherent demodulation of, e.g., the control information. A downlink system with CFI=3 OFDM symbols as control is illustrated in FIG. 3.
From LTE Rel-11 onwards, the above described resource assignments can also be scheduled on the enhanced Physical Downlink Control Channel (EPDCCH). For Rel-8 to Rel-10 only the Physical Downlink Control Channel (PDCCH) is available.
The reference symbols shown in FIG. 3 are the cell specific reference symbols (CRS) and are used to support multiple functions including fine time and frequency synchronization and channel estimation for certain transmission modes.
In the LTE system, a wireless device, such as a user equipment (UE), is notified by the network of downlink data transmission by the physical downlink control channel (PDCCH). Upon reception of a PDCCH in a particular subframe, n, a wireless device is required to decode the corresponding physical downlink shared channel (PDSCH) and to send acknowledgement/negative acknowledgment (ACK/NAK) feedback in a subsequent subframe n+k. The ACK/NAK feedback informs the base station, such as an eNodeB, whether the corresponding PDSCH was decoded correctly. When the eNodeB detects an ACK feedback, it can proceed to send new data blocks to the wireless device. When a NAK is detected by the eNodeB, coded bits corresponding to the original data block will be retransmitted. When the retransmission is based on repetition of previously sent coded bits, it is said to be operating in a Chase combining hybrid automated repeat request (HARQ) protocol. When the retransmission contains coded bits unused in previous transmission attempts, it is said to be operating in an incremental redundancy HARQ protocol.
In LTE, the ACK/NAK feedback is sent by the wireless device using one of the two possible approaches depending on whether the wireless device is simultaneously transmitting a physical uplink shared channel (PUSCH):
If the wireless device is not transmitting a PUSCH at the same time, the ACK/NAK feedback is sent via a physical uplink control channel (PUCCH).
If the wireless device is transmitting a PUSCH simultaneously, the ACK/NAK feedback is sent via the PUSCH.
The LTE Rel-10 standard supports bandwidths larger than 20 mega-Hertz (MHz). One important requirement of LTE Rel-10 is to assure backward compatibility with LTE Rel-8. This should also include spectrum compatibility. This implies that an LTE Rel-10 carrier wider than 20 MHz should appear as a number of LTE carriers to an LTE Rel-8 terminal. Each such carrier can be referred to as a Component Carrier (CC). In particular, for early LTE Rel-10 deployments it can be expected that there will be a smaller number of LTE Rel-10-capable terminals compared to many LTE legacy terminals. One should assure an efficient use of a wide carrier also for legacy terminals, i.e., that it is possible to implement carriers where legacy terminals can be scheduled in all parts of the wideband LTE Rel-10 carrier. A straightforward way to obtain this is by means of Carrier Aggregation (CA). CA implies that an LTE Rel-10 terminal can receive multiple CC, where the CC have, or at least the possibility to have, the same structure as a Rel-8 carrier. CA is illustrated in FIG. 4. A CA-capable wireless device is assigned a primary cell (PCell) which is always activated, and one or more secondary cells (SCells) which may be activated or deactivated dynamically.
The number of aggregated CC as well as the bandwidth of the individual CC may be different for uplink and downlink. Symmetric configuration refers to the case where the number of CCs in downlink and uplink is the same whereas an asymmetric configuration refers to the case that the number of CCs is different. It is noteworthy that the number of CCs configured in a cell may be different from the number of CCs seen by a terminal: A terminal may for example, support more downlink CCs than uplink CCs, even though the cell is configured with the same number of uplink and downlink CCs. Each component carrier operates its own individual HARQ instance.
In typical deployments of WLAN, carrier sense multiple access with collision avoidance (CSMA/CA) is used for medium access. This means that the channel is sensed to perform a clear channel assessment (CCA), and a transmission is initiated only if the channel is declared as Idle. In case the channel is declared as Busy, the transmission is essentially deferred until the channel is deemed to be Idle.
A general illustration of the listen before talk (LBT) mechanism of Wi-Fi is shown in FIG. 5. After a Wi-Fi station A transmits a data frame to a station B, station B shall transmit the ACK frame back to station A with a delay of 16 μs corresponding to a short interframe space (SIFS). Such an ACK frame is transmitted by station B without performing a LBT operation. To prevent another station interfering with such an ACK frame transmission, a station shall defer for a duration of 34 μs (referred to as a distributed coordination function interframe space (DIFS)) after the channel is observed to be occupied before assessing again whether the channel is occupied. Therefore, a station that wishes to transmit first performs a clear channel assessment (CCA) by sensing the medium for a fixed duration DIFS. If the medium is idle then the station assumes that it may take ownership of the medium and begin a frame exchange sequence. If the medium is busy, the station waits for the medium to go idle, defers for DIFS, and waits for a further random backoff period.
In the above basic protocol, when the medium becomes available, multiple Wi-Fi stations may be ready to transmit, which can result in collision. To reduce collisions, stations intending to transmit select a random backoff counter and wait, i.e., backoff, for that number of slot channel idle times. This waiting is the backoff period. A component carrier upon which a CCA is made and for which the delay of transmission is applied is referred to herein as a backoff channel. The amount of the delay is random according to a random backoff counter. The random backoff counter is selected as a random integer drawn from a uniform distribution over the interval of [0, C] where C is a length in integers of a contention window (CW). The random backoff counter establishes the backoff period. The default size of the contention window, CWmin, is set in the IEEE specifications referred to above. Note that collisions can still happen even under this random backoff protocol when there are many stations contending for the channel access. Hence, to avoid recurring collisions, the size of the contention window is doubled whenever the station detects a collision of its transmission up to a limit, CWmax, also set in the IEEE specifications. When a station succeeds in a transmission without collision, it resets its contention window size back to the default value CWmin.
For multi-carrier operation, Wi-Fi follows a hierarchical channel bonding scheme to determine its transmission bandwidth for a frame, which could be 20 MHz, 40 MHz, 80 MHz, or 160 MHz, for example. In the 5 GHz band, wider Wi-Fi channel widths of 40 MHz, 80 MHz, 160 MHz or 80+80 MHz are formed by combining contiguous 20 MHz sub-channels in a non-overlapping manner A pre-determined primary channel performs the CW-based random access procedure after a deferral period if necessary, and then counts down the random number generated. This deferral period is therefore not the same as the backoff period. The secondary channels only perform a quick clear channel assessment (CCA) check for a point coordination function interframe space (PIFS) duration (generally 25 μs) before the potential start of transmission to determine if the additional secondary channels are available for transmission. Based on the results of the secondary CCA check, transmission is performed on the larger bandwidths; otherwise transmission falls back to smaller bandwidths. The Wi-Fi primary channel is always included in all transmissions, i.e., transmission on secondary channels alone is not allowed.
For a device not utilizing the Wi-Fi protocol, EN 301.893, v. 1.7.1 provides the following requirements and minimum behavior for the load-based clear channel assessment.
1) Before a transmission or a burst of transmissions on an Operating Channel, the equipment shall perform a Clear Channel Assessment (CCA) check using “energy detect”. The equipment shall observe the Operating Channel(s) for the duration of the CCA observation time which shall be not less than 20 μs. The CCA observation time used by the equipment shall be declared by the manufacturer. The Operating Channel shall be considered occupied if the energy level in the channel exceeds the threshold corresponding to the power level given in point 5 below. If the equipment finds the channel to be clear, it may transmit immediately (see point 3 below).2) If the equipment finds an Operating Channel occupied, it shall not transmit in that channel. The equipment shall perform an Extended CCA check in which the Operating Channel is observed for the duration of a random factor N multiplied by the CCA observation time. N defines the number of clear idle slots resulting in a total idle period that needs to be observed before initiation of the transmission. This period is referred to as a backoff period and is typically random. Thus, the value of N shall be randomly selected in the range 1 . . . q every time an Extended CCA is required and the value stored in a random backoff counter. The value of q is selected by the manufacturer in the range 4 . . . 32. This selected value shall be declared by the manufacturer (see clause 5.3.1 q of European Telecommunication Standards Institute (ETSI) EN 301 893V1.7.1 (2012-06)). The random backoff counter is decremented every time a CCA slot is considered to be “unoccupied”. When the random backoff counter reaches zero, the equipment may transmit.NOTE 1: The equipment is allowed to continue Short Control Signaling Transmissions on this channel providing it complies with the requirements in clause 4.9.2.3 of ETSI EN 301 893V1.7.1 (2012-06).NOTE 2: For equipment having simultaneous transmissions on multiple (adjacent or non-adjacent) operating channels, the equipment is allowed to continue transmissions on other Operating Channels providing the CCA check did not detect any signals on those channels.3) The total time that an equipment makes use of an Operating Channel is the Maximum Channel Occupancy Time which shall be less than (13/32)×q ms, with q as defined in point 2 above, after which the device shall perform the Extended CCA described in point 2 above.4) The equipment, upon correct reception of a packet which was intended for this equipment, can skip CCA and immediately (see Note 3, below) proceed with the transmission of management and control frames (e.g. ACK and Block ACK frames). A consecutive sequence of transmissions by the equipment, without it performing a new CCA, shall not exceed the Maximum Channel Occupancy Time as defined in point 3 above.NOTE 3: For the purpose of multi-cast, the ACK transmissions (associated with the same data packet) of the individual devices are allowed to take place in a sequence5) The energy detection threshold for the CCA shall be proportional to the maximum transmit power (PH) of the transmitter: for a 23 dBm equivalent isotropically radiated power (e.i.r.p.) transmitter the CCA threshold level (TL) shall be equal or lower than −73 decibel power ratio (dBm)/MHz at the input to the receiver (assuming a 0 dB isotropic (dBi) receive antenna). For other transmit power levels, the CCA threshold level TL shall be calculated using the formula: TL=−73 dBm/MHz+23−PH (assuming a 0 dBi receive antenna and PH specified in dBm e.i.r.p.). An example to illustrate the EN 301.893 LBT is provided in FIG. 6, where X represents a failed CCA, and where, in one embodiment, each CCA of the sequence of CCA checks occupies a 9 micro-second slot.
Up to now, the spectrum used by LTE is dedicated to LTE. This has the advantage that the LTE system does not need to care about the coexistence issue and the spectrum efficiency can be maximized. However, the spectrum allocated to LTE is limited which cannot meet the ever increasing demand for larger throughput from applications/services. Therefore, a new study item has been initiated in 3GPP on extending LTE to exploit unlicensed spectrum in addition to licensed spectrum. Unlicensed spectrum can, by definition, be simultaneously used by multiple different technologies. Therefore, LTE should consider coexistence with other systems such as IEEE 802.11 (Wi-Fi). Operating LTE in the same manner in the unlicensed spectrum as in the licensed spectrum can seriously degrade the performance of Wi-Fi, as Wi-Fi will not transmit once it detects the channel is occupied.
One way to utilize the unlicensed spectrum reliably is to transmit essential control signals and channels on a licensed carrier. That is, as shown in FIG. 7, a wireless device is connected to a PCell in the licensed band and one or more SCells in the unlicensed band. In this application we denote a secondary cell in unlicensed spectrum as licensed-assisted access secondary cell (LAA SCell).
The use of LTE carrier aggregation (CA), introduced since Rel-10, offers a way to increase the peak data rate, system capacity and user experience by aggregating radio resources from multiple carriers that may reside in the same band or different band.
In Rel-13, LAA (Licensed-Assisted Access) has attracted much interest in extending the LTE carrier aggregation feature towards capturing the spectrum opportunities of unlicensed spectrum in the 5 GHz band. WLAN operating in the 5 GHz band already supports 80 MHz in the field and 160 MHz is to follow in Wave 2 deployment of IEEE 802.11ac. Enabling the utilization of multi-carrier operation on unlicensed carrier using LAA is deemed necessary as further CA enhancements. The extension of the CA framework beyond 5 carriers has been started in LTE Rel-13. The objective is to support up to 32 carriers in both UL and DL.
The existing contention window adaptation protocols are based on the reception of a single automated repeat request (ARQ) feedback value (ACK/NACK) that is received after the transmission of a burst of data. In the case of LTE, first a hybrid ARQ (HARQ) protocol is followed instead of a simple ARQ protocol. Thus, multiple retransmissions based on HARQ feedback may be needed before a single ARQ feedback value at the higher layer is generated. Furthermore, in LTE the HARQ feedback is only available after a delay of 4 ms which corresponds to multiple subframes. Existing solutions assume the feedback is available after a very short time interval after the transmission ends. Thus, these solutions do not effectively deal with a system like LTE where the feedback delay is much larger. How to adapt contention window sizes in a multi-carrier setting has also not been defined yet for LAA.
On each LAA carrier, multiple wireless devices may be scheduled for reception or transmission by an eNB in a single subframe. In addition, a single LAA transmission may consist of multiple subframes. Finally, a transmission to or from a single wireless device may have multiple HARQ feedback values if the transmission is a multi-codeword transmission. Thus, there are multiple ways in which multiple feedback values may be received corresponding to a single transmission burst following a successful channel contention. A central problem is how multiple HARQ feedback values corresponding to different component carriers are used in determining the contention window size(s) for the next channel contention.